The latest evidence: The romantic "nuptial flight" in the insect world existed 100 million years ago!

Swarm is a group gathering behavior that occurs during the courtship process of insects. Mayflies, caddisflies, stoneflies, and many lower-level Diptera Longicorn insects all have the habit of swarming. During swarms, male insects release pheromones or sounds to attract female insects, thereby greatly increasing the chances of male insects finding a mate. However, important questions such as the origin of insect swarms and the early evolution of swarm behavior have not been clearly answered.

Figure 1. Reconstruction of the mating flight of caddisflies in the mid-Cretaceous period (drawn by Zuo Xiaoran) shows the scene of Trichoptera insects (caddisflies) performing their mating flight in the evening of the mid-Cretaceous period 99 million years ago.

To address this scientific challenge, Professor Ren Dong and his colleagues from the Ministry of Education Innovation Team on Insect Evolution at Capital Normal University published a research paper in National Science Review. They examined more than 100 amber specimens of insect aggregations from the Middle Cretaceous period of Myanmar and established five new species: Copulariella ramus from the Microcadidae family (1 new genus, 1 new species), Palerasnitsynus queqiaoi, Palerasnitsynus qixi, Palerasnitsynus aggregatus, and Palerasnitsynus xiuqiu.

Figure 2. Specimens of insect aggregations in mid-Cretaceous amber: A, Copulariella ramus; B, Palerasnitsynus queqiaoi; C, Palerasnitsynus qixi; D, Palerasnitsynus aggregatus.

Through systematic specimen inspection, it was found that sexual dimorphism existed in Trichoptera during this period, and a series of important characteristics and differences were discovered in the amber Trichoptera cluster: the male's mouthparts were extremely degenerate, with larger and more prominent compound eyes, the female's enlarged abdomen, specialized wing shape and mating posture of males and females, and differences in the number of males and females in the cluster. At the same time, combined with statistics and measurements of the body parameters of all well-preserved individuals and principal component analysis, it was shown that Trichoptera had evolved nuptial flight behavior in the Middle Cretaceous.

In order to study the evolution of nuptial flight in Trichoptera, the team constructed a phylogenetic tree of Trichoptera using a data set of 16 families of extant Trichoptera based on a research approach combining ancient and modern times. On this basis, combined with the analysis of ancestral state reconstruction, it was concluded that nuptial flight behavior already existed when Trichoptera first appeared in the Triassic, indicating that nuptial flight behavior is the ancestral trait of Trichoptera. In the evolution of Trichoptera, nuptial flight behavior as an ancestral trait has also experienced multiple reversals; in the extant Anclopalmatidae, nuptial flight behavior has been lost in some groups of the Sphenopteridae and Polyspurred Caddisflies; in the extant Entelopalmatidae, nuptial flight is missing in multiple lineages, including Microcaddisfly, Protocaddisfly, Glossodactylidae, Lepidodactylidae, Marshcaddisfly, and Toothed Ceratidae.

Figure 3. Macroevolution of nuptial flight in Trichoptera: The phylogenetic tree of Trichoptera constructed based on morphological and molecular data shows that nuptial flight is the ancestral trait of Trichoptera. Since the Eocene, bats have exerted strong predation pressure on Trichoptera with nuptial flight, resulting in the loss of nuptial flight in many Trichoptera lineages.

Since the Mesozoic Era, birds, pterosaurs and dragonflies have put a lot of pressure on flying insects. However, ancestral state analysis shows that only a few evolutionary branches of Trichoptera lack nuptial flight, suggesting that these flying predators have no obvious impact on the nuptial flight of Trichoptera.

The nuptial flight of living caddisflies mainly occurs at dusk and at night. The nuptial flight of Mesozoic caddisflies is also likely to occur after dusk. The key to whether birds and reptiles are diurnal or nocturnal lies in the shape of the scleral ring composed of a circle of scleral bones surrounding the eyeball. Studies have shown that the shape of the scleral ring of most pterosaurs and Mesozoic birds is similar to that of modern diurnal birds, with a smaller inner diameter. Nocturnal birds such as owls have a larger inner diameter to gather light at night. The body structure of early dragonflies is not much different from that of modern dragonflies, and they are likely to be active during the day. It is speculated that the early birds and large insects in the Mesozoic era did not put much predation pressure on nuptial flying caddisflies. But in the Cenozoic era, nocturnal flying predators - bats - appeared.

Icaronycteris in the Eocene already had the ability to echolocate and hunt at night. Studies on the teeth and stomach contents of Eocene bat fossils show that early bats mainly fed on flying insects. In order to avoid being detected by bats' echolocation, many insect groups have evolved features that can detect ultrasound since the Eocene, such as the tympanic auditory organs of Noctuoidea, Tettigoniidae, and Gryllidae. However, both the living Trichoptera and the Trichoptera in fossils lack this feature, which means that Trichoptera insects do not have the ability to detect bat ultrasound. Field observations show that caddisflies in mating flights are one of bats' favorite foods. The signals generated by a large number of insects gathering during mating flights are easier to detect by ultrasound than those of individual insects. Since the Eocene, it is likely that the existence of bats has exerted strong predation pressure on Trichoptera that have mating flight behavior, resulting in the loss of mating flight behavior in many Trichoptera lineages.

For more details, please read the original article:

Wang, et al. Swarming Caddisflies in the Mid-Cretaceous. National Science Review, 2024, 11: nwae227, doi: 10.1093/nsr/nwae227